<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            color: #999
        }
    </style>
</head>

<body>
    <input type="text" value="手机" name="" id="">
    <script>
        //1.获取元素
        var text = document.querySelector('input');
        //2.注册事件 获得焦点事件 onfocus
        text.onfocus = function() {
            if (this.value == "手机") {
                this.value = "";
            }
            this.style.color = "#333"
                // console.log('得到焦点');
        }

        text.onblur = function() {
            // console.log("失去焦点");
            if (this.value == "") {
                this.value = "手机";
            }
            this.style.color = "#999"
        }
    </script>
</body>

</html>